Pressure-induced large increase of Curie temperature of the van der Waals ferromagnet VI3

Evolution of magnetism in single crystals of the van der Waals compound VI3\nin external pressure up to 7.3 GPa studied by measuring magnetization and ac\nmagnetic susceptibility is reported. Four magnetic phase transitions, at T1 =\n54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed\nat ambient pressure. The first two have been attributed to the onset of\nferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is\ncharacterized by the magnetic ordering transition at Curie temperature TC and\nthe transition between two different ferromagnetic phases TFM, accompanied by a\nstructure transition from monoclinic to triclinic symmetry upon cooling. The\npressure effects on magnetic parameters were studied with three independent\ntechniques. TC was found to be almost unaffected by pressures up to 0.6 GPa\nwhereas TFM increases rapidly with increasing pressure and reaches TC at a\ntriple point at ~ 0.85 GPa. At higher pressures, only one magnetic phase\ntransition is observed moving to higher temperatures with increasing pressure\nto reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization\nis dramatically reduced by applying pressure (by more than 50% at 2.5 GPa)\nsuggesting a possible pressure-induced reduction of vanadium magnetic moment.\nWe discussed these results in light of recent theoretical studies to analyze\nexchange interactions and provide how to increase the Curie temperature of VI3.\n